Mitochondria play a central role in cell metabolism and control multiple aspects of neuronal signaling. By efficiently buffering Ca2+ influx during neuronal excitation and slowly releasing Ca2+ back into the cytosol, mitochondria shape [Ca2+]i transients and regulate Ca2+-dependent neuronal functions, such as excitability, synaptic transmission and gene expression. Ca2+ rise in the mitochondrial matrix stimulates Ca2+-dependent dehydrogenases and boosts ATP production to meet the increase in energy demand during excitation. However, mitochondrial overload with Ca2+ can kill neurons, and mitochondrial Ca2+ dysregulation is implicated in neuronal damage during stroke and in neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Despite the importance of mitochondrial Ca2+ transport to neuronal life and death, the molecules that mediate mitochondrial Ca2+ uptake and release in neurons are not known. This knowledge gap presents a major obstacle in our progress toward understanding and therapeutically correcting mitochondrial functions in neurons. The main objectives of this proposal are to identify molecules that mediate mitochondrial Ca2+ uptake in peripheral and central neurons, and to establish their roles in neuronal Ca2+ signaling, ATP synthesis, synaptic transmission and excitotoxicity. Our preliminary studies indicate that two novel molecules, MCU (CCDC109A) and MCUb (CCDC109B), are broadly expressed in the peripheral and central nervous systems, and that MCU is required for mitochondrial Ca2+ uptake in neurons whereas MCUb inhibits this Ca2+ transport mechanism. Moreover, our pilot data using MCU KO mice showed that MCU loss dramatically, but not completely, reduced mitochondrial Ca2+ uptake, altered Ca2+ signaling and mitochondrial function and provided remarkable protection against glutamate-induced toxicity. Our central hypothesis is that MCU and MCUb play important but opposite roles in the regulation of mitochondrial Ca2+ uptake in neurons, bioenergetics, Ca2+ signaling and synaptic transmission, and that knockout of MCU, but not of MCUb, protects neurons from excitotoxicity and reduces neuronal damage in ischemic stroke. We will employ a multidisciplinary approach involving genetic Ca2+ and ATP sensors, patch-clamp recording, knockout mice and a mouse model of ischemic stroke to test this hypothesis in three specific aims. Aim 1 will establish the roles of MCU and MCUb in mitochondrial Ca2+ transport and Ca2+ signaling in central and peripheral neurons. Aim 2 will examine the impact of MCU and MCUb on presynaptic Ca2+ signaling and synaptic transmission. Aim 3 will establish the roles of MCU and MCUb in excitotoxicity and ischemic stroke. We anticipate that this work will be transformative because it will establish the molecular basis for genetic and pharmacological manipulation of mitochondrial Ca2+ transport in neurons, and may lead to the development of new therapeutics that target mitochondrial Ca2+ uniporters for treating stroke and other neurological disorders associated with excitotoxicity.